The invention relates to a leak detector operating according to the counterflow principle and comprising a first high vacuum pump the entry side of which is connected to the inlet of the leak detector, a second high vacuum pump the entry side of which is connected to a mass spectrometer, a primary pump the entry side of which is connected to the exit sides of the two high vacuum pumps, and a bypass connecting the inlet of the leak detector to the primary pump and including a first valve.
A leak detector of this kind is described in EP 0 283 543 A1. It serves to detect leaks in a vacuum-tight apparatus. To this end, a light test gas is introduced into the apparatus to be tested and this apparatus is placed in a vacuum-tight room from which the available gas is sucked off. Alternatively, test gas can be sprayed from outside onto a test piece the interior volume of which is connected to the inlet of the leak detector. In the sucked-off gas, constituents of the test gas can be detected by a mass spectrometer and be evaluated. If there are test gas portions, this suggests a leak in the apparatus. The leak detector according to the counterflow principle referred to comprises a primary pump and a first high vacuum pump which are operated in series. A second high vacuum pump connects the mass spectrometer to the entry side of the primary pump. In a suction stage, the way via the first high vacuum pump is blocked by closing a valve arranged upstream of this pump and the container including the test piece is pumped dry via a bypass including a valve. When the pressure in the container has decreased below a value of about 100 mbar, a throttle valve in the conduit leading to the first high vacuum pump is opened so that test gas may flow through the first high vacuum pump. If, in the course of this, test gas (helium) enters into the conduit system, it enters into the second high vacuum pump in the counterflow and through the latter into the mass spectrometer. If helium has not been detected by the mass spectrometer in this stage, a further leak detection with higher sensitivity is started by releasing the connection of the test piece to the first high vacuum pump through another valve of a large nominal diameter. The preevacuation valve in the bypass is closed then. Thereafter, the test piece is evacuated to a pressure of about 10−4 mbar, leakage rates in the order of about 10−5 to 10−10 mbar 1/s being detectable.
Because of the valves arranged in front of the entry side of the first high vacuum pump, this pump is able to operate continuously at full speed under vacuum conditions. The valves arranged upstream, however, have a flow resistance and thus a throttling effect. Therefore, the full suction capacity of the high vacuum pump is not available at the test connection in a non-throttled manner. Particularly, the helium suction capacity of the high vacuum pump cannot be utilized to the full extent thereof which would be convenient to achieve minimum response times.